This invention relates to magnetic heads for tape recorders and the like, and more particularly to a method of preventing corrosion of the magnetic cores forming such a magnetic head.
Heretofore, Permalloy (a trademark of Western Electric Co. for an alloy composed approximately of 78% Ni, 0.3% Mo and the remainder Fe) and ferrite have been employed as core materials for magnetic heads. These materials have the following contrasting characteristics: The Permalloy is superior in magnetic characteristics, but is low in wear resistance; while the ferrite is high in wear resistance, but is inferior in magnetic characteristics because it is magnetically saturated in a short time. On the other hand, alloys known in Japan by the names "Sendust" and "Alperm" are well known in the art as magnetic core materials which are superior in magnetic characteristics and high in wear resistance. "Sendust" is an alloy composed approximately of 5 to 11% Si, 3 to 8% Al and the remainder Fe, while "Alperm" is an alloy composed approximately of 13 to 17% Al and the remainder Fe. However, these materials still have a drawbacks that, unlike the Permalloy and the ferrite, they are inferior in anticorrosive characteristic. In other words, the Permalloy shows an excellent anticorrosive characteristic under any circumstance because it has about an 80% Ni content and about a 20% Fe content. The ferrite also shows an excellent anticorrosive characteristic because it is a stable oxide.
"Sendust" and "Alperm" are generally considered to have good anticorrosive characteristics; however, when they are placed in a high humidity atmosphere or water drops are allowed to drop thereon, a local corrosion called "pitting corrosion" sometimes occurs. The reason for this is believed to be that it is impossible to manufacture the aforementioned alloys which are completely uniform; that is, in the manufacture of the alloys segregation is caused to a slight extent and impurities contained therein form cores. ("Sendust" and "Alperm" show excellent anticorrosive characteristics when they are alloys which are physically completely uniform.) The "Sendust" and "Alperm" are alloys which are low in galvanic potential order and have large amounts of Al and Fe the potential difference of which is relatively large. Accordingly, if completely uniform alloys are not obtained due to the aforementioned segregation or impurities contained therein, minute galvanic cells are formed at the portions of the alloys where the segregation has occurred or the impurities are contained, because of the potential difference between Al and Fe. Accordingly, the minute galvanic cells are discharged through the water films or water drops formed in a high humidity atmosphere surrounding the alloys, as a result of which local corrosions, namely, pitting corrosions are caused. If this pitting corrosion is caused, the density of oxygen contained in the water film or water drop becomes non-uniform, as a result of which a battery action due to the non-uniform oxygen density is effected. Accordingly, the pitting corrosion is advanced at a high rate; that is, a recess which is several tens of micrometers both in diameter and in depth is formed in several tens of minutes to several hours.
In the case where the "Sendust" or "Alperm" is employed for manufacturing magnetic heads, water drops or water films may be allowed to adhere to the magnetic head due to the variations of the environmental conditions, thereby causing pitting corrosion in the magnetic head. If the pitting corrosion does not occur at the gap of the magnetic head or in the vicinity of the gap, the corrosion will not be a problem. However, if the pitting corrosion is caused at the gap of the magnetic head or in the vicinity of the gap as shown in FIG. 1, the magnetic head can no longer maintain its initial performance, because while the width of the gap is, in general, several micrometers, the size of the pitting corrosion is extremely large as was described before.
"Sendust" and "Alperm" being alloys containing Al, Fe, Si, etc., these elements having different dissolution temperature and specific gravity, it is considerably difficult to manufacture an alloy free of segregation and interposed impurities. This is one of the significant reasons why "Sendust" or "Alperm" has not been employed for manufacturing magnetic heads even though superior in magnetic characteristics and wear resistance.